The table was set in three divisions, for Joseph, for his brethren, and for the Egyptians. The sons of Jacob did not venture to eat of the dishes set before them, they were afraid they might not have been prepared according to the ritual prescriptions--a punishment upon Joseph for having slandered his brethren, whom he once charged with not being punctilious in the observance of the dietary laws. The Egyptians, again, could not sit at the same table with the sons of Jacob, because the latter ate the flesh of the animals to which the former paid divine worship.
Joseph and Divine Judgment of Egyptians
Legends of the Jews 1:244